Microchannel plates (MCPs) are thin wafers of electrically conducting glass which contain thousands of open channels or tubes. Each channel diameter is on the order of 10 to 15 microns. The plates are used for the amplification of x-rays, ions, or electrons. They are an essential component of a variety of electronic devices, including night vision goggles.
The principles of microchannel plate fabrication and operation are described in detail in the literature. For example, see an article by Michael Lampton in Scientific American, 245, 62-71 (1981). A fabrication procedure in current use is described in detail in U.S. Pat. Nos. 4,629,486 (Uchiyama et al.) and No. 4,112,170 (Rauscher). The former is particularly concerned with alkali lead silicate cladding glasses; the latter with barium borosilicate core glasses.
Briefly, the method, as there described, involves fusing solid rods of a leachable core material, preferably glass, within tubular pieces of relatively non-leachable skin glass. A bundle of such composite bodies is formed and fusion sealed. The bundle is then drawn down, cut into lengths, rebundled and further drawn. Ultimately, a composite article is obtained in which an interconnected glass matrix of the skin glass encases an array of leachable core elements. The composite is then exposed to a leachant, for example, hydrochloric or nitric acid, to remove the cores. This leaves the skin glass matrix with an array of channels corresponding to the array of core elements. The channels may be on the order of ten microns in diameter. The perforated plate, thus formed, is then heated in a hydrogen-containing atmosphere to produce a surface layer of reduced metal on the channel walls.
My recently issued U.S. Pat. No. 5,034,354 discloses a family of alkali-free, lead silicate glasses that are adapted to perforated plate production. This glass family is composed essentially, in percent by weight, of 28-40% SiO.sub.2, 42-55% PbO, 6-14% BaO, 2-8% ZnO, 1-5% CaO, 0-2% MgO, 0-5% Al.sub.2 O.sub.3 and 0-1.5% Sb.sub.2 O.sub.3.
With these glasses, as with the earlier alkali-containing glasses, it is necessary to fire the perforated plate in hydrogen. This reduces a layer of lead on the channel wall surfaces to obtain the required surface resistivity. This hydrogen reduction adds a further step, requiring special equipment, which it would be desirable to avoid. Also, there is a growing concern about working with lead-containing materials, especially where volatilization might occur.
Consideration has been given to the use of semiconducting glasses instead of reduced lead glasses. J. D. Mackenzie in "Modern Aspects of the Vitreous State", Vol. III (1964) defines a semi-conducting glass as "a homogeneous, single phased and noncrystalline solid in which (a) bulk electrical conduction at a given temperature occurs predominantly by electrons (and/or holes) rather than ions, and (b) the temperature coefficient of resistivity is negative." The literature abounds with examples of such glasses based on transition metal oxides, such as vanadium oxide, and on standard glasses containing special additives, such as bismuth oxide.
Previously known semiconducting glasses have not proven adaptable to the processing required in perforated plate production. Therefore, it is a basic purpose of the present invention to provide a family of semiconducting glasses that is adapted to such use. Another purpose is to provide a novel family of lanthana-iron borate glasses that are semiconducting. A further purpose is to provide such glasses that are resistant to devitrification.